1. Field of the Invention
This invention relates in general to earth boring machines and in particular to improved means to make-up or break-out drill stem members of such machines.
2. Description of the Prior Art
There are commercially available machines which utilize hydraulic cylinders to forcibly move in a linear, upward or downward direction a power swivel, which is commonly driven by a hydraulic motor and pump. When adding or subtracting a drill stem member to the total string of such members, the power swivel is rotated while back-up tongs or equivalent hold a portion of the drill string to cause make-up or break-out of a selected rotary connection in the string. When the connection is broken, for example, the lead of the threads of the drill stem members forces the power swivel in one linear direction or another. Thus, the hydraulic cylinders used to control the linear direction and speed of the swivel must move linearly with the rotational speed of the swivel. Otherwise, the mechanism may strip the threads.